The use of isolators has been necessary to improve the stability of the microwave source in the presence of varying loads and to extend the life of the source of microwave energy, commonly a magnetron. The presence of varying loads results in an impedance mismatch that causes some of the energy to be reflected toward the source of microwave energy. When this energy reaches the tube it may, if of sufficient amplitude progressively destroy it or at least reduce considerably its lifetime. The energy reflected also causes a higher VSWR in the waveguide resulting in a peak electric field that can cause arcing unless the waveguide is of sufficient height.
In certain applications, microwave ovens for instance, it is very important to use an isolator that is compact, lightweight and that can be manufactured at the lowest possible cost. In conventional isolators, the power reflected by the load is absorbed by a resonance process in the ferrite material. The ferrite is heated and must be cooled efficiently to prevent its degradation. These isolators are bulky and use a heavy horseshoe-shaped permanent magnet to provide the required magnetic field. To reduce cost and weight, microwave ovens use a circulator to protect the magnetron. However, the circulators have a high isolation which prevents the mode stirrer from pulling the magnetron to achieve better heat uniformity, an important consideration in microwave heating.